Filters and duplexers used for mobile communications such as smartphones and mobile phones employ acoustic wave devices such as surface acoustic wave devices including an Interdigital Transducer (IDT) formed on a piezoelectric substrate. The acoustic wave device including the IDT has a problem of undesired lateral-mode waves. Japanese Patent Application Publication No. 7-22898 (Patent Document 1) discloses the use of an apodized IDT to reduce undesired lateral-mode waves. Japanese Patent Application Publication No. 2011-101350 (Patent Document 2) discloses making the width of an electrode finger in an edge region of an overlap region of the IDT greater than the width of the electrode finger in a center region of the overlap region to reduce undesired lateral-mode waves. Japanese Patent Application Publication No. 2012-186808 (Patent Document 3) discloses adding a dielectric film or a metal film to the electrode finger in the edge region to reduce undesired lateral-mode waves. Japanese Patent Application Publication Nos. 2006-333024, 9-162679, and 2006-203778 disclose providing a region, the extension direction of the electrode finger in which differs from the extension direction of the electrode finger in the remaining region of the overlap region, to the overlap region of the IDT.
The technique disclosed in Patent Document 1 increases the number of dummy electrode fingers, leading to bad area efficiency and increase in cost. The techniques disclosed in Patent Documents 2 and 3 do not sufficiently reduce undesired lateral-mode waves, and/or increase cost due to an additional fabrication step such as a step of forming a film.